Ethylene copolymers having a density of from about 0.91 g/cm.sup.3 to about 0.96 g/cm.sup.3 can be produced, as disclosed in European patent publications Nos. 4645, 4647 and 12148, by continuously copolymerizing ethylene with one or more higher alpha olefin monomers via a low pressure gas phase reaction. In order to obtain copolymers having the desired density in accordance with the process of these references, however, it is necessary for the polymerization mixture to contain a relatively high proportion of the higher alpha olefin monomer(s) vis-a-vis the ethylene monomer. This is especially true when low density copolymers (i.e., no greater than 0.94 g/cm.sup.3) are desired, e.g., for film and injection molding applications. However, because these higher comonomers polymerize at a slower rate than ethylene, the high concentration of such comonomers in the reactor causes the polymerization rate to decrease and polymer productivity to decline.